1. Field of the Invention
The present invention relates to a picture coding apparatus and a picture coding method for high efficiency encoding of source picture data.
2. Description of Related Art
A picture coding apparatus according to the related art is described in MPEG-2 Test Model 5 as defined in ISO/IEC JTC1/SC29/WG11/N0400 and described briefly below.
FIG. 16 is a block diagram of an image encoder 300 according to MPEG-2 Test Model 5. As shown in FIG. 16, a subtractor 301 obtains the difference between the input video (the source video data) and previously encoded and then decoded picture data. A DCT (discrete cosine transform) converter 302 then converts the difference data obtained by subtractor 301 to frequency domain data, and a quantizer 303 quantizes the frequency domain data passed from DCT converter 302. A variable length coder (VLC) 304 removes redundancy from the quantized data. Buffer 305 smooths and outputs the VLC output from VLC 304 to the transmission path at a certain rate. A dequantizer 306 dequantizes the quantized data from quantizer 303, and an inverse DCT converter 307 inverts the dequantized data from dequantizer 306. Adder 308 then adds the output from inverse DCT converter 307 and the decoded data from n frames before. Note that the data added by adder 308 is hereafter referred to “locally decoded data.”
An in-loop frame memory 309 stores the locally decoded data. Motion compensator 310 controls reading from frame memory 309 using a motion vector, the motion vector indicative of the change detected between the source picture data and the locally decoded data. A quantization controller 311 controls the quantization step, and thus controls the bit rate and the image quality of encoded pictures. An activity calculator 312 calculates activity from the source picture data by obtaining the average of the 64 pels in each 8×8 pel block in a frame or field luminance signal, subtracting this average from the pel value of each of the 64 pels, and obtaining the integral of the difference values.
The MPEG-2 standard defines a general coding method known as the Main Profile. Before encoding in the Main Profile, pictures are rearranged from display order to coding order (this step is not shown in the figures), and are coded according to the picture type. There are three picture types: I-pictures (intraframe predictive-coded pictures), P-pictures (forward motion-compensated prediction pictures), and B-pictures (forward/backward motion-compensated interpolated pictures) Methods of accomplishing the Main Profile are well known from the literature, including The Journal of the Institute of Television Engineers of Japan, Vol. 49, No. 4, pp. 435–466 (1995) and others. Methods for controlling the bit rate in. the above-noted Test Model 5 include (1) target data size for the picture, (2) buffer fullness feedback control, and (3) a quantization step based on activity in the source picture data.
FIG. 17 is a block diagram of a conventional picture coding apparatus 320 as taught in Japanese Patent Laid-open Publication (kokai) 11-234668. Shown in FIG. 17 are encoder 321 such as the above-described H.26X or MPEG encoder; prefilter 322; pel count converter 323; and pel count conversion controller 324 for generating a pel count conversion control signal correlated to the filter frequency control signal generated by encoder 321. Based on the code size produced by encoder 321, picture coding apparatus 320 adjusts the frequency of prefilter 322 and drives pel count conversion controller 324 to select the smallest number of pels required at that frequency.
FIG. 18 is a block diagram of a conventional picture coding apparatus 330 as taught in Japanese Patent Laid-open Publication (kokai) 7-107462. Shown in FIG. 18 are encoder 331 such as the above-described H.26x or MPEG encoder; filter 332; adaptive control circuit 333 for controlling the pass-through characteristic of the filter; and prefilter controller 334 for generating a control signal correlated to the data volume produced by the encoder 331. Based on the data volume produced by the encoder 331, this picture coding apparatus 330 controls the pass-through characteristic of the filter from particular local data detected by adaptive control circuit 333 in the picture.
FIG. 19 is a block diagram of a conventional picture coding apparatus 340 as taught in Japanese Patent Laid-open Publication (kokai) 5-103317. Shown in this figure are encoder 341 such as the above-described H.26X or MPEG encoder; delay 342 for delaying the source picture data; difference calculator 343 for obtaining the difference (distortion) between the source picture data delayed by delay 342 and locally decoded data; and quantization parameter controller 344 for controlling the quantization process using the difference data obtained by difference calculator 343 as a control parameter.
It should be noted that other examples of the related art can be found in the following Japanese Patent Laid-open Publications (kokai): 2000-23162; 11-234668; 11-164305; 10-108197; 10-108167; 10-98712; 9-23423; 8-242452; 7-107462; 6-6784; 5-103317; 4-306094; 3-256484; and 63-304769.
Problems that the Invention is to Solve
Motion compensated interframe coding techniques such as MPEG-2 are conceived primarily for application with digital broadcasting and transmission, such as SDTV and HDTV, and storage media, and in broadcast satellite and terrestrial broadcasting where HDTV is the main, a quite low bit rate (20 Mbps or less) is anticipated (see The Journal of the Institute of Image Information and Television Engineers, Vol. 53, No. 11, pp. 1456–1459 (1999)).
Furthermore, MPEG-2 and the conventional picture coding apparatus 80 [sic] are basic control models, and do not provide sufficient image quality Various quantization control methods have therefore been proposed. When a conventional HDTV signal is compression coded according to the MPEG-2 standard, the bit rate satisfying broadcast quality standards based on ITU-R evaluation methods is 22 Mbps or higher (see The Journal of the Institute of Image Information and Television Engineers, Vol. 53, No. 11, pp. 1456–1459 (1999)).
From this article it is obvious that the bit rate must be further reduced (that is, video compression efficiency improved) in order to achieve a single frequency network (SFN) in terrestrial broadcasting media. However, further reducing the bit rate using conventional control methods necessarily requires larger quantization steps, which are known to increase block distortion and create undesirable visual effects.
Picture coding apparatus 320 described above narrows the passband of the prefilter 322 when the encoder 321 produces a large amount of data, broadens the passband when less data is produced, and converts the data to the smallest necessary number of pixels based on the selected filter frequency. However, the amount of data output by the encoder is a result of coding differences between frames, and even if the spatial frequency is controlled, it is often not possible to have any effect.
Furthermore, picture coding apparatus 330 described above is basically a filter 332 process using only data from analyzing the pels around the filtered pel, and is therefore not always able to improve compression efficiency. More particularly, when the compression rate is greatly increased the bandpass frequency used for visually important areas is also limited, often with adverse visual effects.
Furthermore, picture coding apparatus 340 described above can be expected to be effective at relatively high bit rates because it changes the quantization step distribution based on the detected image difference (distortion). At low bit rates, however, the quantization steps are larger overall.
An object of the present invention is therefore to resolve these problems of the related art.